Water-swellable polymers are used as constituents in sanitary materials which absorb body fluids such as: sanitary napkins, incontinent devices and disposable baby diapers. Examples of such polymers are disclosed in U.S. Pat. Nos. 3,926,891, 4,190,562 and 4,293,609, herein incorporated by reference.
The high level of water-soluble polymers is of concern in such materials because the extractable polymers may be leached from the absorbent structure by aqueous fluids which lessens the efficiency of the product. A high level of water-solubles can cause low water-absorbing capacity in the adsorbent device. Thus, when these water-swellable polymers are contacted with catamenial blood, urine, or other body fluids, they have low initial absorption rates, less effective absorption and become sticky. Therefore, a high water-soluble polymer content can inhibit the absorptive capacity of the absorbing device.
Various processes are known for producing the absorbent polymers such as: bulk polymerization, aqueous solution polymerization, spray polymerization, inverse emulsion polymerization, inverse suspension polymerization and the like.
Generally, the water-swellable, crosslinked, hydrophilic polymers are prepared by the radical copolymerization of an aqueous solution of an appropriate monomer and a crosslinking monomer. The resulting hydrous gel is dried and then pulverized to put it in the form which is useful for incorporation into devices used for the absorption of human fluids. The water-soluble, non-crosslinking monomer of the acid type may be polymerized in two forms. First, the polymerization may be performed on a carboxylic acid monomer in its acid form, and secondly the polymerization can be carried out on the carboxylic acid monomer in its neutralized form, preferably an alkali metal salt form.
When the polymerization is performed on the acid form of the monomer it is necessary to neutralize the resultant polymer in order to impart absorbent properties and make it compatible with human applications. Such neutralization has been carried out after the monomer has been polymerized into a viscous gel which is difficult to handle. The neutralization is accomplished by contacting the viscous gel with an amount of basic solution or material effective to neutralize the acid groups present in the polymer. The neutralization is impaired due to the difficulty in mixing the neutralizing agent evenly into the polymerized gel.
When using the monomers in the acid form the crosslinking monomer is soluble in the aqueous acid monomer and therefore allows for uniform mixing of the crosslinking monomer and the aqueous acid monomer. The uniform mixing aids in the formation of a polymer which is evenly and uniformly crosslinked. This may affect the amount of water-soluble polymers which are present in the absorbent polymer product. In effect, the uniform distribution of the crosslinking monomer may reduce the amount of water-soluble polymer which may leach out of the absorbent polymer when contacted with the fluid to be absorbed.
However, if the monomers are polymerized in a neutralized form, such as in a salt form, the crosslinker may not be soluble in the monomer solution and will therefore not be uniformly distributed in the monomer solution. The lack of uniform distribution may increase the concentration of water-soluble polymers present in the absorbent polymer.
U.S. Pat. No. 4,286,082, herein incorporated by reference, discloses a process for preparing an absorbent polymer composition by polymerizing an aqueous solution of a mixture of an alkali metal acrylate and acrylic acid and a crosslinking monomer in the presence of a surface-active agent. The surface-active agent acts to uniformly disperse the crosslinking monomer in the alkali metal salt and acrylic acid solution. However, the presence of a surface-active agent in the resultant polymer can inhibit the desired wicking ability of aqueous fluids through absorbent structures containing the polymer. The surface-active agent reduces the surface tension of the fluid to be absorbed resulting in an absorbent capacity of the absorbent device which is not fully utilized.
Absorbent devices often contain a wicking fibrous material which serves as an aid in the distribution of the fluid to be absorbed by the device. Examples of such wicking fibrous materials include cellulose batting, paper, woven or non-woven cloth, cellulosic fluff and the like. The wicking material distributes the fluid throughout the device so that ideally all of the absorbent polymer in the device is contacted with the fluid. This results in an efficient use of the absorbent polymer which is in the device. A capillary action between the fluid and the wicking material results in the distribution of the fluid. Such action is dependent on the surface tension of the fluid which is being distributed. Surface-active agents function to reduce the surface tension or the interfacial tension between two liquids when dissolved in water or water solutions. This reduction in surface tension can have the effect of reducing the capillary action between the fluid and the wicking material. This results in an under-utilization of the absorbent capacity of the absorbent device. Therefore the preferred dispersants are those which do not adversely affect the wicking action of the absorbent device.
An absorbent polymer composition prepared from a neutralized monomer solution which does not contain a surface-active agent which impedes capillary action in absorbent articles would be desirable. Minimizing the extractable content of the polymer composition in aqueous fluids is also desirable. Further, preparing the polymer in the absence of a surface-active agent enables one to better utilize the full absorbent capacity of the absorbent device in which the polymer is a constituent. Thus, a process for the polymerization of a neutralized monomer solution without using a surface-active agent to obtain a polymer low in extractables is desired. Such polymers should desirably exhibit good gel strengths, high rates of absorption, low levels of water-soluble polymer, and high capacities for absorbing fluids.